Data transmission using digital modulation is becoming more and more widely used in radio beams. The digital radio beams that have been developed so far have had a heterodyne structure like analog radio beams. However, technological development now makes it possible to design modulators and demodulators that operate directly at microwave frequencies. This makes it possible to implement low cost transmission systems.
An article by J. P. Bonin entitled (in translation) "Microwave demodulation" published in "L'Onde Electrique", January 1986 (Vol. 66, No. 1), describes the general structure of a microwave demodulator for digital radio links using quadrature amplitude modulation (QAM). The article concludes by stating that increasing digital data rate (n.times.34 Mbit/s, 140 Mbit/s) requires 16-QAM to be adopted. The demodulator proper can be developed very quickly. However, in the absence of appropriate additional equipment, it is very vulnerable to difficulties associated with propagation. It needs to be associated within the highest-performance devices for matching a transmission channel, in particular by recombination space diversity and by time equalization.
The use of QAM which is of ever-increasing complexity and which has a high degree of spectrum efficiency, makes radio beams more sensitive to manufacturing dispersion in the characteristics of each component assembly or subassembly. In particular, the error rate on a radio link is increased by demodulator quadrature error and by variation thereof as a function of temperature.
An object of the invention is to mitigate this drawback.